1. Technical Field
The invention relates to the art of brakes for heavy-duty vehicles. More particularly, the invention relates to brake shoes of drum brakes for heavy-duty vehicles. Still more particularly, the invention is directed to a sealing interface that is disposed between a brake shoe table and a brake shoe lining by the application of a zinc-rich coating to the shoe table. The sealing interface resists the formation of corrosion cells on the brake shoe table, which in turn reduces the build-up of significant rust between the brake shoe table and the brake shoe lining, thereby preventing cracking and potential dislodging of the lining, while also facilitating replacement of linings when they eventually wear.
2. Background Art
Heavy-duty vehicles, such as trucks and tractor-trailers or semi-trailers, always include brake assemblies that enable the vehicles to stop when traveling. In many cases, these brake assemblies are of the drum brake type. Each drum brake typically includes a pair of brake shoes, and each brake shoe in turn includes a sacrificial, high-coefficient of friction brake lining that is mounted on a metal backing plate, which is known as a shoe table. In the prior art, certain road conditions have created an undesirable condition for brake shoes, which is known in the art as rust jacking.
More particularly, when a heavy-duty vehicle travels over roads and there is moisture on the road surface, road spray may be generated. Road spray is the moisture that is caused to move upward from the road surface by the vehicle tires toward the underside of the vehicle. In addition, cross splash may be generated, which is the splashing of moisture from puddles toward the underside of the vehicle when a vehicle tire contacts a puddle. By contacting the underside of the vehicle, road spray and cross splash contact many exposed components on the lower portion of the vehicle. Because the brake shoes are located on the lower portion of the vehicle and typically are unprotected from road spray and cross splash, when the vehicle travels over wet roads, road spray and cross splash tends to collect on the brake shoes. For the purpose of convenience, reference below shall be made to road spray with the understanding that such reference includes both road spray and cross splash.
It is known in the art that road spray may include salt or other chemicals that are present on the road surface. For example, salt from salt water is often present on roads near a sea, and roads in areas with abundant ice or snow are often treated with certain chemicals for anti-icing or de-icing. Anti-icing and de-icing shall collectively be referred to herein for the purpose of convenience as anti-icing. Such anti-icing chemicals include sodium chloride, calcium chloride, magnesium chloride, and mixtures thereof. In the past, sodium chloride, which is commonly referred to as road salt, had typically been used to treat roads for anti-icing. However, more aggressive anti-icing chemicals have been developed, including calcium chloride and magnesium chloride, each of which will be described in greater detail below. Because road spray includes such salt or other chemicals, when road spray collects on the brake shoes, the salt and/or other chemicals also collect on the brake shoes. The collection of salt and/or anti-icing chemicals, and in particular the more aggressive anti-icing chemicals, has created a condition known in the art as rust jacking.
As mentioned above, in each brake shoe, a brake lining is mounted on a shoe table. To provide secure mounting, the brake lining typically is attached to the shoe table by mechanical fasteners such as rivets, bolts, or the like. However, the use of mechanical fasteners enables small gaps to form in areas of the interface between the brake lining and the shoe table that are between or outside of the fasteners, including the perimeter or the outside edges of this interface. When road spray with salt and/or chemicals collects on the brake shoes, the moisture and salt and/or other chemicals accumulate in the gaps at the perimeter or the side edges of the interface between the brake lining and the shoe table.
In addition, the shoe table is formed with openings that receive the mechanical fasteners which secure the brake lining to the shoe table. In many cases, each shoe table is designed to accommodate several different types of brake linings, each one of which may include a different pattern for the mechanical fasteners. As a result, when the brake lining is attached to the shoe table, the shoe table may include openings that have not received a mechanical fastener. Such un-used openings allow moisture and salt and/or other chemicals to weep or pass through the shoe table and again accumulate at the interface between the brake lining and the shoe table.
The accumulation of moisture and salt and/or other chemicals at the interface between the brake lining and the shoe table causes corrosion cells to form on the shoe table at this interface. The corrosion cells often begin to form on the shoe table at the gaps that are at the perimeter or the side edges of the interface between the brake lining and the shoe table, and at areas in the interface adjacent to un-used fastener openings in the shoe table. The corrosion cells then propagate or spread to other gaps at the interface, and/or to other areas in the interface that are adjacent to un-used shoe table fastener openings. Rust then builds up at the corrosion cells, and once the rust buildup becomes significant, it pushes the brake lining outward from the shoe table, which is a condition known in the art as rust jacking.
In the art, while the formation of any noticeable rust creates the potential for rust buildup and eventual rust jacking, significant rust buildup is generally understood to be a thickness of rust that is sufficient to adversely affect the structure and/or the operation of the brake shoe, as will be described in greater detail below. It is to be understood that reference herein to significant rust buildup is made in such a context, and by way of example, includes an amount of rust that is typically in a thickness range of from about 0.05 inches to about 0.20 inches, or enough rust to form a discrete flake. It is to be further understood that the actual thickness of rust that constitutes significant rust buildup is often dictated by the construction of the specific brake lining that is employed, as some brake linings are capable of accommodating or tolerating more rust buildup than others.
Because the brake lining is secured to the shoe table by mechanical fasteners, once the rust buildup becomes significant, the rust creates an outward force on the brake lining against the fasteners which may cause the brake lining to crack. Once a brake lining cracks, it is no longer useable, and the brake shoe must then be replaced to ensure proper functioning of the vehicle brakes. In some cases, the rust buildup may be extensive enough to dislodge the lining from the mechanical fasteners, requiring immediate replacement of the brake shoe. As a result, rust jacking undesirably reduces the life of the brake shoe, which undesirably increases the cost, time and effort associated with maintaining the vehicle.
Traditionally, brake shoe tables had been coated with water-based paint, which was applied by dip painting, or dipping the brake shoe tables into the paint. Such water-based dip painted brake shoe tables were often able to resist rust jacking when sodium chloride was used to treat roads for anti-icing. However, as described above, calcium chloride has been developed as an anti-icing chemical that is more aggressive than sodium chloride. With the use of calcium chloride as an anti-icing chemical, water-based dip painting has been ineffective in providing a sufficient resistance to the formation of corrosion cells on the shoe table at the interface between the brake lining and the shoe table. Without sufficient resistance to the formation of corrosion cells, rust can build up and rust jacking can occur on such brake shoes having water-based dip painted brake shoe tables.
In addition, severe braking conditions experienced by the vehicle often exacerbate rust jacking in vehicles that employ brakes having water-based dip painted brake shoe tables. More particularly, when drum brakes of a heavy duty vehicle are applied in a severe braking condition, such as a sudden stop or an extended stop, there is slight movement of the brake lining and the shoe table relative to one another. This movement causes the brake lining to scuff the shoe table at the interface between the brake lining and the shoe table, which removes some of the paint of a water-based dip painted brake shoe table. As a result, the bare metal of the shoe table is exposed in the scuff areas. Moisture and salt and/or chemicals that have accumulated at the interface between the brake lining and the shoe table are then able to form corrosion cells in the scuff areas on the shoe table, which may in turn enable rust to build up and lead to rust jacking.
In order to reduce rust jacking on brake shoes having water-based dip painted brake shoe tables, and particularly when calcium chloride is employed as an anti-icing chemical, an alternative process known as electro-coating or e-coating of the brake shoe table was developed in the prior art. In e-coating, an epoxy-based or acrylic-based coating is deposited onto the shoe table in a bath, and an electric current is introduced into the bath to promote a surface reaction on the brake shoe, which enables optimum depositing of the coating. This optimum depositing of the epoxy-based or acrylic-based coating resists the formation of corrosion cells by calcium chloride on the shoe table at the interface between the brake lining and the shoe table. As a result, in situations where anti-icing chemicals that include calcium chloride collect on the brake shoes, e-coating of the brake shoe has been found to reduce rust jacking.
However, the e-coating process involves the purchase and maintenance of costly equipment, causing the process to be undesirably expensive. The e-coating process is also a relatively sensitive process. More particularly, if the conditions and steps of the process, which are known to those skilled in the art, are not performed under optimum conditions, the resulting e-coating may not sufficiently adhere to the surface of the shoe table. If the e-coating does not sufficiently adhere to the shoe table, the coating may separate from the shoe table under performance conditions, which enables corrosion cells to form on the shoe table, in turn creating the possibility that rust jacking still may occur.
In addition, magnesium chloride was developed as an aggressive anti-icing chemical, and has been used alone and in combination with other chemicals. When anti-icing chemicals that include magnesium chloride collect on the brake shoes, e-coating does not provide a sufficient resistance to the formation of corrosion cells on the shoe table at the interface between the brake lining and the shoe table. Without sufficient resistance to the formation of corrosion cells, rust is able to build up and rust jacking is able to occur on brake shoes with such e-coated brake shoe tables.
Moreover, as described above, severe braking conditions cause the brake lining to scuff the shoe table at the interface between the brake lining and the shoe table. This scuffing removes some of the e-coating, resulting in the exposure of the bare metal of the shoe table. Moisture and salt and/or other chemicals that have accumulated at the interface between the brake lining and the shoe table are then able to form corrosion cells in the scuff areas on the shoe table, which may in turn enable rust to build up and lead to rust jacking.
Also, as mentioned above, the brake lining is a sacrificial component that wears out over time, and is intended by manufacturers to be replaced at certain intervals. In order to reduce cost and waste in this replacement, it is desirable to remove the worn brake lining from the shoe table and install a new brake lining on the shoe table, thus re-using the shoe table. The structural strength and durability of a shoe table typically enables the shoe table to be re-used multiple times, thereby desirably reducing the cost of brake repair or replacement. Typically, a principal factor that undesirably limits the number of times a shoe table can be re-used is corrosion or pitting of the shoe table, which is a disadvantage with a brake shoe having an e-coated shoe table. That is, when a shoe table that was previously e-coated is re-used, the coating typically is not sufficient to continue to resist the formation of corrosion cells on the shoe table at the interface between the brake lining and the shoe table, thereby enabling rust jacking to occur. Moreover, if the shoe table is e-coated again, surface imperfections on the shoe table caused by typical wear or previous surface pitting prevent optimum depositing of the coating, also reducing the ability of the coating to resist the formation of corrosion cells on the shoe table at the interface between the brake lining and the shoe table, again enabling rust jacking to occur.
In order to overcome the above-described disadvantages associated with e-coating, other processes were developed in the prior art. One of these prior art processes involves applying coatings to the shoe table that are cured by ultraviolet (UV) light to resist the formation of corrosion cells on the surface of the shoe table at the interface between the brake lining and the shoe table. While such UV-cured coatings sometimes reduce rust jacking, they are often undesirably expensive to apply, and in many cases, do not provide sufficient resistance to the formation of corrosion cells on the shoe table at the interface between the brake lining and the shoe table for a significant length of time, thereby eventually enabling rust jacking to occur anyway. Moreover, when severe braking conditions cause the brake lining to scuff a shoe table with a UV-cured coating, the scuffing removes some of the coating, resulting in the exposure of the bare metal of the shoe table. Moisture and salt and/or chemicals that have accumulated at the interface between the brake lining and the shoe table are then able to form corrosion cells in the scuff areas on the shoe table, which may in turn enable rust to build up and lead to rust jacking.
Another prior art approach to reducing rust jacking when aggressive anti-icing chemicals, such as magnesium chloride, are employed has been to apply a double-sided sheet of adhesive to the shoe table before the brake lining is attached. The adhesive sheet is a high-temperature adhesive composed of an acrylic/polymer material or a urethane-based adhesive material. The adhesive contacts the shoe table and the brake lining in an attempt to impede the passage of moisture and salt and/or chemicals to the interface between the brake lining and the shoe table.
However, as described above, the brake lining is a sacrificial component that wears out over time, and is intended by manufacturers to be replaced at certain intervals. In order to reduce cost and waste in this replacement, it is desirable to remove the worn brake lining from the shoe table and install a new brake lining on the shoe table, thus re-using the shoe table. As mentioned above, it is desirable to re-use a shoe table multiple times in order to desirably reduce the cost of brake repair or replacement. In brake shoes that employ the double-sided sheet of adhesive, the adhesive attachment to both the shoe table and the brake lining makes removal of a worn brake lining from the shoe table extremely difficult and time consuming. As a result, removal of the worn brake lining from the shoe table cannot be performed efficiently or cost-effectively, often resulting in the entire brake shoe being scrapped, which undesirably increases waste and the cost to maintain the heavy-duty vehicle.
As a result, there is a need in the art for a heavy-duty vehicle brake assembly that overcomes the disadvantages of the prior art by providing an economical and effective sealing interface between the shoe table and the brake lining that resists the formation of corrosion cells on the brake shoe table, which in turn reduces the build-up of significant rust between the shoe table and the brake lining, thereby preventing cracking and potential dislodging of the lining, while also facilitating cost-effective replacement of worn linings.
The heavy-duty vehicle brake assembly with a sealing interface of the present invention satisfies these needs, as will be described in detail below.